1. Field of the Invention
The present invention relates to a sputtering apparatus used for manufacturing optical discs, comprising a reflective film made of a metal material such as aluminum deposited upon a disc substrate and, in particular, to a sputtering apparatus which is capable of successively carrying out sputtering-treatment of a disc substrate.
2. Description of the Prior Art
Optical discs which are formed for enabling given information signals such as audio or video signals to be recorded or reproduced have been proposed. The optical disc comprises a disc substrate of a material such as polycarbonate or acrylic resin, having bits or grooves formed thereon, and which is coated with a reflective film made of metal material such as aluminum.
Methods of coating a disc substrate with a metal material include vacuum deposition, ion plating and sputtering methods.
The vacuum deposition and ion plating methods include the steps of heating a metal material (evaporation source) to evaporate the metal material in a vacuum chamber and exposing a disc substrate to the vapor of the metal material for depositing the metal material on the substrate. Mass production of optical discs by these methods relies upon a so-called batch manufacturing system in which a multiplicity of optical discs are manufactured by one time evaporation of the metal material by using a large vacuum chamber which is capable of collectively containing a plurality of optical discs. Therefore, the manufacturing system does not only become more complicated in structure and larger in size, but also the resultant products become non-uniform in quality when located in different positions in the vacuum chamber.
In contrast to this, the sputtering method has advantages that treatment is completed in a short period of time, that successive treatments are possible and that coating of individual disc substrates can be performed under the same conditions. Accordingly, adoption of this sputtering method can not only provide the substrates with uniform coating, but the method can also cope well with mass production and multi-kind production.
The sputtering treatment comprises flowing a discharge gas, such as argon, at a low pressure into a vacuum chamber in which disc substrates and a metal material (target), such as aluminum, are disposed. An electric field is established in the vacuum chamber to ionize the discharge gas and ejecting atoms and molecules out of the metal material with the ionized discharge gas. That is, in the vacuum chamber, the discharging gas is ionized and collides with the metal material. The metal material is thus sputtered so that the sputtered metal material is deposited upon the disc substrates to form thin films thereon.
A sputtering apparatus for performing this sputtering treatment has been proposed which is designed to successively perform the sputtering treatments on a plurality of disc substrates by rotating a disc-like carrying table on which the plurality of disc substrates are placed in a circumferential direction as shown in, for example, FIG. 1.
The sputtering apparatus is designed so that the inner pressure is kept at a given vacuum degree and has a vacuum chamber 103 which is formed with a disc entrance and exit opening 101 through which disc substrate 100 is loaded and unloaded and a sputtering treatment portion 102 for performing sputtering treatments. The disc entrance and exit opening 101 is provided with a lid 104 for keeping an inner pressure in the vacuum chamber 103, which is supported and is moved toward and away from the vacuum chamber 103.
On the other hand, the sputtering treatment portion 102 has a disc treatment opening 105 which is formed similarly to the disc entrance and exit opening 101 and a cylindrical treatment portion 106 mounted on the Vacuum chamber 103 so that the portion 106 covers the upper side of the disc treatment opening 105. A metal material (target) 107 which is sputtered on the substrate 100 is disposed in the treatment portion 106, and argon gas and the like are charged therein at a low pressure.
A disc-like carrying table 108 which successively carries a plurality of the disc substrates 100 between the disc entrance and exit opening 101 and the sputtering treatment portion 102 is disposed in the vacuum chamber 103. The carrying table 108 is supported on a rotary shaft 108a projecting in the vacuum chamber 103 and is driven by drive means (not shown) to rotate around the axis thereof. The carrying table 108 is formed with a circular positioning recess 108b on which the disc substrate 100 rests. A tray 109 which positions the disc substrate 100 by aligning a centering hole 100a of the disc substrate 100 with the recess 108b is adapted into the recess 108b. The tray 109 is formed into a disc having a diameter slightly larger than the outer diameter of the disc substrate 100 and is provided at the substantial center thereof with a projection 109a which passes through the chucking centering hole of the disc substrate 100. A through-hole 108c through which push up rods 110 and 111 for pushing up the tray 109 pass is formed in the substantial center of the positioning recess 108b of the carrying table 108.
The push up rods 110 and 111 pass through through-holes which are opposite to the disc entrance and exit opening 101 and the sputtering treatment portion 102, respectively and extend to the inside of the vacuum chamber 103, and are designed to be moved in a vertical direction by a drive apparatus (not shown). When the disc substrate 100 is carried to positions opposite to the disc entrance and exit opening 101 and the sputtering treatment portion 102, the respective push up rods 110 and 111 are moved upward for urging the trays 109 and 109a upon the peripheries of the disc entrance and exit opening 101 and the disc treatment opening 105, respectively so that the vacuum chamber 103 is sealed from the outside (atmosphere). When unloading of the disc substrate 100 from the opening 101 and sputtering treatment in the sputtering treatment portion 102 is completed, the respective push up rods 110 and 111 are moved toward the bottom of the vacuum chamber 103.
In the thus formed sputtering apparatus, the plurality of disc substrates 100 supplied from the disc entrance and exit opening 101 are successively carried to the sputtering treatment portion 102 at which they are successively subjected to sputtering treatment and simultaneously with this, unloading of the sputter-treated disc substrates 100 and loading of the untreated disc substrates 100 onto the carrying table 108 is carried out.
Removal of a sputter-treated disc substrate and supply of the untreated new disc substrate D to the carrying table 108 is achieved while the tray 109 is brought into a pressure-contact with the peripheral edge of the disc entrance and exit opening 101 by the push up rod 110. The vacuum chamber 103 is sealed by a seal member, such as an O-ring, provided along the peripheral edge of the tray 109 so that the atmosphere is prevented from being introduced into the vacuum chamber 103. After completion of these operations, the disc entrance and exit 101 is closed by the lid 104 and simultaneously with this, the vacuum chamber 103 is evacuated to a given vacuum pressure suitable for sputtering by operating a vacuum pump.
Since the carrying table 108 is disc-like in shape and the distance between the upper side of the carrying table 108 and the vacuum chamber 103 is very short, evacuation particularly above the disc substrate cannot be sufficiently achieved since the vacuum degree in the vacuum chamber 103 cannot be made uniform. If the vacuum degree is insufficient in such a manner, residual impurities, such as water or gas, remaining in or on the disc substrate D are scattered and deposited on a sputtering electrode and the like, such that an excellent film may not be formed.
Due to the fact that the tray has a very heavy weight and that the pressure in the sputtering treatment portion 102 is as high as about 130 kg/m.sup.2, deterioration of the O-ring (not shown) for sealing the space between the push up rod 111 for supporting the tray 109 and the vacuum chamber 103 is considerable.